The present invention relates to a pressure tank.
Recent years have seen the development of hydrogen engine automobiles and electric fuel cell automobiles to reduce the amount of carbon dioxide emitted from vehicles. In electric fuel cell automobiles, power is generated by electrochemical reaction of hydrogen and oxygen. The power is supplied to the motor to produce motive power. A pressure tank for the storage of hydrogen is generally installed in electric fuel cell vehicles. An example of such a pressure tank is disclosed in Japanese National Phase Laid-Open Patent Publication No. 2000-504810. The pressure tank disclosed in Japanese National Phase Laid-Open Patent Publication No. 2000-504810 is provided with a metal or resin liner and a shell. The shell covers the outer surface of the liner and functions to ensure pressure resistance of the pressure tank.
In general, when gas is charged into a pressure tank, the temperature within the pressure tank rises due to the compression heat of the gas. When the shell is formed of a carbon fiber reinforced plastic (CFRP), the heat within the pressure tank is not radiated to the exterior due to the poor heat conductance of the CFRP. The amount of gas that can be charged into the pressure tank decreases as the temperature within the pressure tank increases. Particularly, when gas is quickly charged into the pressure tank, the temperature within the pressure tank rises significantly. As a result, a sufficient amount of gas cannot be loaded into the pressure tank.
One means for increasing the amount of gas that may be charged into a pressure tank is to accommodate an absorption material, which is capable of absorbing gas and releasing the absorbed gas, in the pressure tank. Such a material includes, for example, a hydrogen absorption alloy (metal hydrate) capable of absorbing hydrogen and releasing the absorbed hydrogen. The hydrogen absorption reaction by a hydrogen absorption alloy is an exothermic reaction. Accordingly, when a hydrogen absorption alloy is used, the heat generated by the hydrogen absorption reaction is added to the compression heat. This fact must be considered when designing the pressure tank.
A pressure tank is generally provided with a heat exchanger for maintaining the temperature in the pressure tank within a predetermined temperature range. The heat exchanger exchanges heat between the interior of the pressure tank and a heat transfer medium, which circulates within a heat transfer tube, to maintain the temperature in the pressure tank within a predetermined range. In conventional pressure tanks, the heat transfer tube extends through the liner of the pressure tank out of the pressure tank. In order to ensure the hermetic seal of the pressure tank, the heat transfer tube is brazed or welded to the liner. However, the brazing or welding of the heat transfer tube to the liner reduces the strength of the liner strength, and particularly, the fatigue strength. Therefore, conventional pressure tanks do not have satisfactory durability.
Japanese Laid-Open Patent Publication No. 2000-249425 discloses first and second pressure tanks incorporating heat exchangers. The first pressure tank has heat insulation case accommodating the heat exchanger and hydrogen absorption alloy. The heat insulation case is supported in the pressure tank by a support member, which contacts the interior surface of the pressure tank in points or in a linear manner. The second pressure tank is provided with a stainless steel container 151 and a housing case 152, which is accommodated in the container 151, as shown in FIG. 12. The housing case 152, which has a plurality of fine holes 152a, accommodates a heat exchanger 153 and hydrogen absorption alloy powder. A gas permeable heat insulation material 154, such as glass wool, is arranged in the space defined between the inner surface of the container 151 and the outer surface of the housing case 152. The heat exchanger 153 includes a heat transfer tube 155, through which a heat transfer medium is circulated, and a plurality of fins 156. The heat transfer tube 155 extends through the housing case 152 and is supported at opposite ends of the container 151. That is, the housing case 152 and the hydrogen absorption alloy are supported by the container 151 through the heat transfer tube 155.
The pressure tank is installed into electric fuel cell automobiles and hydrogen engine automobiles as a hydrogen storage tank for storing hydrogen fuel. In this case, it is desirable that the hydrogen capacity of the pressure tank be as large as possible. It is further desirable that the weight of the pressure tank be as light as possible. The hydrogen absorption alloy significantly increases the hydrogen capacity of the pressure tank. However, the hydrogen absorption alloy expands when absorbing hydrogen. Therefore, the pressure tank must have sufficient strength to withstand the added stress caused by the expansion of the hydrogen absorption alloy. This leads to an increase in the weight of the pressure tank.
One means of eliminating this problem is to support only one end or two ends of a hydrogen absorption unit, which is provided with a heat exchange function and includes a hydrogen absorption alloy, with the pressure tank. In this case, the pressure in the pressure tank may be adjusted regulate the amount of hydrogen charged into the empty space of the pressure tank. However, when only one end or two ends of the hydrogen absorption unit is supported by the pressure tank, a large load is applied to the portion of the hydrogen absorption unit supported by the pressure tank and the portion of the pressure tank supporting the hydrogen absorption unit. These portions must be reinforced. However, this would increase the weight of the pressure tank instead of reducing the weigh of the pressure tank.
In the first pressure tank disclosed in Japanese Laid-Open Patent Publication No. 2000-219125, stress tends to be concentrated at certain portions of the pressure tank during the expansion of the hydrogen absorption alloy because of the point contact or linear contact of the support member relative to the interior surface of the pressure tank. In the case of the second pressure tank shown in FIG. 12, the heat insulation material 154 is used for insulating heat and not for supporting the housing case 152. Assuming that the housing case 152 is to be supported by the heat insulation material 154, the heat insulation material 154 would have to be filled into the space defined between the outer surface of the housing case 152 and the inner surface of the container 151 to the extent that there would be no space for charging hydrogen.